**Core Concept**
Gene duplication is a crucial mechanism in the evolution of organisms, allowing for the creation of new genes and genetic variation. This process can lead to the emergence of novel gene functions, protein structures, and eventually, new species. Gene duplication is particularly important in the evolution of messenger RNA (mRNA), as it enables the creation of new genes with different regulatory elements and coding sequences.

**Why the Correct Answer is Right**
The correct answer, **mRNA**, is the primary product of gene expression, responsible for carrying genetic information from DNA to the ribosome for protein synthesis. Gene duplication of mRNA-encoding genes can lead to the creation of new genes with different functions, allowing for increased genetic diversity and adaptation to changing environments. This process can occur through various mechanisms, including DNA replication errors, recombination, and transposition. The duplicated gene can then evolve into a new gene with distinct regulatory elements, coding sequences, or both, leading to the emergence of new gene functions and protein structures.

**Why Each Wrong Option is Incorrect**
**Option B:** rRNA (ribosomal RNA) is a structural component of the ribosome, essential for protein synthesis, but gene duplication of rRNA-encoding genes is not directly involved in the evolution of new gene functions or protein structures.

**Option C:** tRNA (transfer RNA) is responsible for transferring amino acids to the ribosome during protein synthesis, but gene duplication of tRNA-encoding genes is not a primary mechanism for creating new gene functions or protein structures.

**Option D:** hnRNA (heterogeneous nuclear RNA) is a precursor to mRNA, but gene duplication of hnRNA-encoding genes is not directly involved in the evolution of new gene functions or protein structures.

**Clinical Pearl / High-Yield Fact**
Gene duplication can lead to the creation of new genes with distinct regulatory elements, coding sequences, or both, allowing for increased genetic diversity and adaptation to changing environments. This process is particularly important in the evolution of new gene functions and protein structures, contributing to the emergence of new species.

**✓ Correct Answer: A. mRNA**